The decision to apply an antitachycardia therapy, and the choice of which therapy to apply (e.g., shock or ATP stimulation) is operated by an algorithm that performs a detection and classification of the various tachyarrhythmia according to several criteria, mainly according to the ventricular frequency, but also according to the stability of the ventricular intervals, the stability of atrio-ventricular conduction, the mode of starting or chamber of origin of the tachycardia, etc. (see in particular the EP-A-0 626 182 and its corresponding U.S. Pat. No. 5,462,060 and EP-A-0 838 235 and its corresponding U.S. Pat. No. 5,868,793 commonly assigned herewith to ELA Médical).
The ventricular frequency is the first criterion that makes it possible in particular to distinguish three situations relevant to the tachycardia detection and classification algorithm:                1. A detected frequency that is lower than a given frequency threshold, called “frequency of detection of VT (ventricular tachycardia)”, for example, about 140 bpm. The aforementioned algorithm considers that this ventricular rate is slow, but is not pathological, and never justifies the application of an antitachycardia therapy.        2. A detected frequency which is between the frequency of detection of VT, typically 140 bpm, and a higher frequency called the “frequency of detection of VF (ventricular fibrillation)”, typically at about 200 bpm. The algorithm considers in this range that there is “suspicion of VT” and carries out a more thorough analysis, implementing criteria other than just the ventricular frequency. This more thorough analysis proceeds to determine more precisely the type of disorder and to decide whether it is necessary to apply a therapy, and if so what therapy (shock or ATP stimulation) to apply;        3. A detected frequency that is higher than the frequency of detection of VF, typically 200 bpm. The algorithm considers in this case that the application of a therapy is always necessary and to be delivered without delay.        
The above-mentioned implantable devices typically include, in addition to the antitachycardia therapy means described, means (e.g., detection circuits, stimulation circuits, and associated control logic) for providing antibradycardia stimulation therapy. Antibradycardia stimulation allows, as with a traditional demand implantable pacemaker, delivery if necessary of stimulation pulses to the ventricle (and possibly to the atrium) in the absence of a detected spontaneous depolarization of the relevant cavity.
Antibradycardia stimulation is preferably operated at a variable frequency, depending on the activity of the patient. In this regard, the pacemakers can be equipped with one or more suitable activity sensors that may be physiological (for example, minute-ventilation sensor) or physical (for example, acceleration), as known in the art, making it possible to evaluate the instantaneous cardiac output requirements and/or physical activity level of the patient and to control consequently the stimulation frequency in a rate responsive manner. This is known in the art as pacing using a function of enslavement (to the patient activity) or rate responsive pacing. Thus, when the patient is exerting an “effort”, i.e., engaging in activity other than resting, the activity sensor provides a corresponding measure that is used by the control logic to increase the stimulation frequency (and to lower the given frequency as the effort level is reduced accordingly). This enables the patient to support hemodynamically the greater effort. Of course, this variable stimulation frequency is typically provided with a maximum limit, a value known as the “maximum stimulation frequency”. This limit value is typically programmed by the physician at the time of the implantation of the device or during follow-up visits by the patient.
It has been discovered that a particular situation arises when the maximum stimulation frequency for antibradycardia can be programmed at a value that is higher than the threshold frequency of detection of the VT for antitachycardia. In this case, for high values of the stimulation frequency, the device considers that this high rate is a sinusal tachycardia (ST), because the rate is in 1:1 association, stable and without acceleration. But it can happen that a VT begins while at the same time the patient is in a situation of effort where the patient is stimulated by the device at a relatively high frequency, undergoing antibradycardia therapy.
The frequency of the VT and that of stimulation can be close so that atrial stimulation can mask the spontaneous R waves. This might lead to a delay in the detection of the VT or to an under-detection of the VT. As a result, there can be a less than optimal delivery of the antitachycardia therapy that would be appropriate. Indeed, it may lead to a delay in antitachycardia therapy that can reach several minutes.
Of course, to avoid this issue, physicians have been known to program the maximum stimulation frequency to a value that is lower than the frequency of detection of VT. But this has the disadvantage of limiting the maximum frequency at which the patient can be appropriately stimulated. For example, in the solution above, where the frequency of detection of VT is set at 140 bpm, this has resulted in limiting the maximum stimulation frequency to 140 bpm. It would be desirable, however, particularly for younger patients, to increase the maximum stimulation frequency to allow them to obtain comfortably a greater level of effort, for example, up to 180 bpm.